1. Field of the Invention
The invention relates to a driving force transmission apparatus.
2. Discussion of Background
Japanese Patent Application Publication No. 2003-014001 (JP 2003-014001 A) describes a conventional driving force transmission apparatus that is mounted in, for example, a four-wheel-drive vehicle, and in which a pair of rotary members are coupled to each other by a clutch so that torque is transmittable between the rotary members.
The driving force transmission apparatus includes a first rotary member, a second rotary member, a friction main clutch, an electromagnetic clutch, a friction pilot clutch, and a cam mechanism. The first rotary member rotates together with an input shaft. The second rotary member is rotatable about the axis of the first rotary member. The friction main clutch couples the second rotary member and the first rotary member to each other so that torque is transmittable therebetween. The electromagnetic clutch is arranged next to the main clutch along the axes of the first rotary member and the second rotary member. The friction pilot clutch is driven upon reception of the electromagnetic force of the electromagnetic clutch. The cam mechanism receives rotational force from the first rotary member due to the clutch action of the pilot clutch, and converts the rotational force into pushing force toward the main clutch.
The first rotary member is formed of a cylindrical front housing and an annular rear housing. The front housing is open at both ends. The rear housing is fitted to a rear opening portion of the front housing. The first rotary member is coupled to the input shaft. The first rotary member is configured to rotate upon receiving the driving force of an engine from the input shaft.
The second rotary member is arranged so as to be rotatable relative to the first rotary member about its rotation axis. The second rotary member is coupled to an output shaft.
The main clutch includes inner clutch plates and outer clutch plates, and is arranged between the first rotary member and the second rotary member. The main clutch is configured such that the inner clutch plates and the outer clutch plates frictionally engage with each other to couple the first rotary member and the second rotary member to each other so that torque is transmittable therebetween.
The electromagnetic clutch is arranged along the axes of the first rotary member and the second rotary member. The electromagnetic clutch is configured to generate electromagnetic force to drive the pilot clutch.
The pilot clutch includes inner clutch plates and outer clutch plates, and is arranged between the main clutch and the electromagnetic clutch. The pilot clutch is configured to apply the rotational force of the first rotary member to the cam mechanism.
The cam mechanism has a pushing portion that applies pushing force to the main clutch through cam action caused by the rotational force from the first rotary member. The cam mechanism is arranged between the first rotary member and the second rotary member.
With the above configuration, when driving force from the engine is input into the first rotary member via the input shaft, the first rotary member rotates about its axis. When current is supplied to the electromagnetic clutch, the pilot clutch is driven by the electromagnetic force of the electromagnetic clutch.
Subsequently, when the cam mechanism receives rotational force from the first rotary member when the pilot clutch is driven, the rotational force is converted into pushing force by the cam mechanism, and the pushing force is applied to the main clutch.
Then, the inner clutch plates and the outer clutch plates of the main clutch approach each other and frictionally engage with each other, and the first rotary member and the second rotary member are coupled to each other through the frictional engagement so that torque is transmittable therebetween. In this way, the driving force of the engine is transmitted from the input shaft to the output shaft via the driving force transmission apparatus.
With the driving force transmission apparatus described in JP 2003-014001 A, while the four-wheel-drive vehicle travels in a two-wheel-drive mode, the cam mechanism receives not only rotational force from the second rotary member but also rotational force from the first rotary member due to so-called drag torque that occurs on the basis of the viscosity of the lubricating oil between the inner clutch plates and the outer clutch plates of the pilot clutch, and the pushing portion of the cam mechanism pushes the main clutch by cam thrust force generated by the rotational force from the first rotary member. Therefore, the main clutch receives pushing force that is amplified by the cam mechanism, and the inner clutch plates and the outer clutch plates of the main clutch frictionally engage with each other. As a result, not only turning performance and fuel economy are adversely influenced but also desirable clutch action of the clutch is not obtained when the four-wheel-drive vehicle shifts from the two-wheel-drive mode to a four-wheel-drive mode.
In order to suppress the above-described adverse influence due to drag torque, for example, the clearance between adjacent two clutch plates of the main clutch may be increased. In this case, however, the response of clutch action of the main clutch decreases.